Picture display device

ABSTRACT

A display device ( 1 ) comprises a luminescent display screen ( 15 ), and means for directing electrons towards the display screen ( 15 ). Said means comprises comprise an arrangement of at least three plates ( 36, 41, 42 ), with a middle plate ( 36 ) having selection apertures, and selection electrodes associated with the selection apertures. The selection apertures, the selection electrodes and the plates are arranged for having the electron currents, on their way from a source to the electroluminescent screen, selectively pass the apertures in the middle plate and alternately run at opposite sides of the middle plate. An anti-leakage layer (44 eg) for providing in operation a tunneling counteracting potential is provided on a plate of the arrangement to prevent leakage electrons through gaps between the middle plate and an adjacent plate.

This invention relates to a display device comprising a vacuum envelopean inner side of which is provided with an luminescent display screen,said vacuum envelope comprising at least an electron source and meansfor directing electrons towards the display screen, said meanscomprising an arrangement of branched electron ducts, the display devicecomprising an arrangement of at least three plates, comprising a middleplate having selection apertures, and selection electrodes associatedwith the selection apertures for application of selection voltages, theselection aperture, the selection electrodes and the plates arranged forhaving the electron currents on their way from the source to theelectroluminescent screen selectively pass the apertures in the middleplate and alternately run at opposite sides of the middle plate.

An embodiment of such a display device is known from U.S. Pat. No.5,781,166. In the known display device, electron transport between theelectron source (for example a wire cathode) and the cathode-luminescentscreen takes place by means of electron transport ducts. Electrons aretransported from an entrance to an exit by applying a voltage across thetransport duct.

A simplified, explanatory description of the electron transportmechanism is that electrons impinging on the wall of the transport ductgenerate secondary electrons. Due to the applied voltage, electrons mayimpinge on the wall of the electron transport duct, thereby generatingthe secondary electrons. An electron current is thus formed in thedirection of propagation of the electron-transport duct.

The known device comprises an arrangement of at least three plates,comprising a middle plate having apertures, the electrons on their wayto the electroluminescent screen selectively passing through theselection apertures in the middle plate and being directed at oppositesides of the middle plate.

For example, an electron current injected into one part of a transportduct, running at one side of the middle plate, can exit said part of thetransport duct via two or more apertures, which lead the electroncurrent to an entrance of a further part of the transport duct extendingat the opposite side of the middle plate, which part has at the exitside again two or more exit apertures which lead to a further part of atransport duct at the first mentioned side of the middle plate. In thismanner a branched network of transport ducts is formed.

In the known device the transport ducts are defined by and in anarrangement of three plates. Subsequent parts of the transport ducts arepreferably formed at either side of the middle plate, the connectionbeing formed by the selection apertures in the middle plate. Such athree-plated arrangement has the advantage that a very compact electrontransport structure is possible.

However, the inventors have found that the amount of electron currentexiting a transport duct at a designated position is generally less thanthe amount of current injected into a transport duct. This leads to adegradation of the quality of the image produced on the display screen.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a display device of the typementioned in the opening paragraph with an improved image rendition.

To this end, the display device is characterized in that an anti-leakagelayer is provided for enabling, in operation, an electric potential on aplate of the arrangement, said electric potential counteractingtunneling of electrons between the middle plate and an adjacent plate.

An arrangement with three plates, in which electron ducts alternatelyrun at either side of the middle plate, has as a consequence that duringoperation an electric potential along the middle plate is formed. Theinventors have realized that this electric potential can cause electronsto tunnel in between plates. “Tunneling”, within the concept of theinvention, indicates unwanted transport of charge, usually throughcracks or slits in between plates.

Electrons can for instance leak to a selection electrode or to aneighboring transport duct. Also, the electric potential may cause fieldemission, so that electrons are made (for example from an electrode) andfind their way to another electrode or transport duct.

Any of these “tunneling” mechanisms has a number of effects. First ofall, current is lost, since electrons tunnel in between the plates andare lost. Also, some current will find its way from a part of atransport duct at a certain level to a higher level part of a transportduct, even if this was not the intention. Furthermore, some current willhit the selection electrodes causing a current to run in the selectionelectrodes and finally, apart from a diminution of the “intended”current and thereby of the current hitting the display screen at theintended position, there are also currents hitting the screen atunintended positions, after having tunneled in between the plates to atransport duct which should have been closed.

The current diminution and related effects are caused by randomprocesses and are thus relatively unpredictable. However, application ofanti-leakage layers according to the present invention is an efficientsolution for the current diminution problem. Such layers provide anelectric potential which, in operation, counteracts the tunneling of theelectrons.

In a first embodiment of the invention the anti-leakage layer is formedby a passive anti-leakage layer. Within the concept of the invention“passive anti-leakage layers” are understood to comprise layers whichare in operation not connected to a potential source.

In a second embodiment the anti-leakage layer is formed by an activeanti-leakage layer, which within the concept of the invention is to beunderstood to comprise a conductive layer with connections for beingconnected to a potential source, by which means a potential barrieragainst tunneling of electrons may be made.

The passive layers of the first embodiment have the advantage that onceprovided they function without application of any additional electricpotentials. Disadvantages of such passive layers are formed by the factthat they are based on some charge-up effect, which will take some timeto build up, and inherent conductivity of materials will cause someleaking away of the accumulated charges, which will cause a (usuallyvery) small leaking effect. Also the “stopping power” of the passiveanti-leakage layers is set and given once they are applied, there islittle flexibility.

The active layers of the second embodiment has the advantage that aflexible barrier can be made that is able to prevent leakage currentsmore efficiently. However, the active layers require the application ofan electric potential.

Within the concept of the invention a single device may comprise bothtypes of layers.

BRIEF DESCRIPTION OF THE DRAWINGS

These and further aspects of the invention will be described in greaterdetail, by way of example, and with reference to the accompanyingdrawings, in which:

FIG. 1 shows a display device in accordance with the present state ofthe art;

FIGS. 2A and 2B show schematically a detail of a display device of FIG.1;

FIG. 3 shows an arrangement of three plates as known from the deviceshown in FIG. 1;

FIGS. 4A to 4C schematically show an arrangement of three platesillustrating the basic principle of the invention;

FIGS. 5A to 5E illustrate a first embodiment of a display deviceaccording to the invention and

FIGS. 6A to 6E illustrate a second embodiment of a display deviceaccording to the invention.

The drawings are schematic and generally not drawn to scale, and likereference numerals generally refer to like parts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a display device 1 in accordance with the present state ofthe art.

The Figure corresponds to FIG. 2 of U.S. Pat. No. 5,781,166 to whichreference is also made for further details. The contents of U.S. Pat.No. 5,781,166 are hereby incorporated by reference. The display devicecomprises a network 10 of transport ducts (here within also called“electron ducts”) which branches at a point 13. An electron currentwhich is introduced into the network 10, in this case at point 11, issubsequently directed towards the exits 12 via a large number ofelectron junctions 13 (hereinafter also referred to as “currentjunctions”) interconnected by transport ducts 14.

At each of said electron junctions 13, the electron current is led, inthis example, in one of two directions. The current junctions can beregarded as the nodes of the network 10. In this example, the exits forma two or three-dimensional array and the nodes of the network also forma two or three-dimensional array. If the projection of the electroncurrents on a plane parallel to the display screen is considered, thenthe electron current distribution is two-dimensional. When the electronshave exited the exits 12 they are directed to a display screen 15. Thedisplay device itself in contained in, or comprised in an evacuatedenvelope.

FIG. 2A is a top view of the branched network 10. The electron currenttravels from the entrance 21 to exits 22, for example exit 12 a, viaeight current junctions 23 a to 23 h. The exits 22 form atwo-dimensional array, as do the current junctions. FIG. 2B is a moredetailed view of a current junction (in this example 23 f). Each currentjunction or node in the network comprises, in this example, anelectron-supply duct 24 a and two electron-exhaust ducts 25 a and 25 b,apertures 26 a and 26 b forming the connections between the supply andexhaust ducts and control electrodes 27 a and 27 b which can beenergized so as to selectively guide the electrons into one of the twoexhaust ducts 25 a or 25 b via the connecting apertures 26 a or 26 b,respectively. Viewed from the entrance 11, the bias on the electrodesincreases in the direction of the exits 22.

For example, the voltage on the electrodes in front of the currentjunction 23 a is several tens to several hundreds of volts higher thanthe voltage in front of the entrance 11, the voltage on the electrodesin front of the current junction 23 b is several tens to severalhundreds of volts higher than the voltage on the electrodes in front ofcurrent junction 23 a, etc., etc. This results in an electric fieldbeing applied across the transport ducts between the current junctions,said field ensuring that electrons entering via the entrance travelthrough the network. The electron current is controlled by applying avoltage higher than the normal bias voltage to one of the electrodes, avoltage lower than the bias voltage preferably being applied to theother electrode. As a result, the electrons are led into the exhaustduct associated with said one electrode. Calculating from the entrance21, a number can be assigned to a node or current junction 23 in thenetwork, which corresponds to the number of current junctions betweenthe relevant current junction and the entrance plus one. In FIG. 2A, thecurrent junction 23 a is a current junction of the first order, currentjunction 23 b is a current junction of the second order, currentjunction 23 f is a current junction of the sixth order. Preferably,electrodes of current junctions of equal order are interconnected. Byvirtue thereof, the number of electrical connections is reduced. In thisexample, electrodes associated with current junctions of equal order areinterconnected. The electrodes of current junction 23 f are, forexample, connected to electrodes of current junction 23 f′. In FIG. 2A,the path along which a current entering via entrance 21 is guided toexit 22 a via the network is represented by a thick line. The overallnumber of exits is formed by an array of 16 times 16=256 pixels. Onecontrol electrode is required to control the intensity of the electroncurrent which is injected into the network via the entrance 21, andsixteen electrodes are required to drive this array. The total number ofcontrol electrodes is 17. The same array of pixels in the known displaydevice comprises 32 control electrodes (i.e. 16+16). For an array of2^(n) pixels, the known display device needs n+m control electrodes,wherein n*m=2^(n), i.e. it is at least 2*2^(n/2), whereas the displaydevice as shown in FIGS. 1 and 2A, 2B comprises (2n+1) controlelectrodes. Consequently, the number of control electrodes is small. Byvirtue thereof, the display device is relatively simple. FIGS. 1 and 2A,2B show an embodiment in which the distance from the entrance apertureto the corresponding exit aperture, through the network, issubstantially the same for each of the exit apertures.

The network is simple if the number of exits is equal to n^(m), whereinn is an integer greater than 1 and m is an integer greater than 1. Anetwork can then be used having m nodes between the entrance and everyexit, each node comprising a supply duct and n exhaust ducts. In theexamples n is chosen to be two, but obviously, nodes having more thantwo exhaust ducts fall within the scope of the invention. Two exhaustducts per node has the advantage that the network can be driven by meansof binary codes (a binary code can be assigned to each pixel) and thatthe nodes are simple too.

FIG. 3 shows some details of a preferred embodiment of a display devicein accordance with the invention. FIG. 3 shows a transport duct beingformed by three plates 36, 41, 42. The middle plate 36 is arrangedbetween upper (or lower) plate 41 and lower (or upper) plate 42. Abranched network is formed by transport ducts 31 a,b in upper plate 41and transport ducts 32 a,b in lower plate 42.

In operation, electrons enter the branched network through an aperture34 in the upper plate 41. By suitable application of the electric field,the electrons move in either direction through transport duct 31 a, andare drawn through one of the apertures 35 a in the middle plate 36,towards the lower plate 42. Arriving at the lower plate 42, theycontinue in either direction through one of the transport ducts 32 a.

The electron transport continues in the same manner, so that theelectrons successively pass an aperture 35 b in the middle plate 36, areguided through a transport duct 31 b, pass an aperture 35 c in themiddle plate 36 and are guided through a transport duct 41 b. From atransport duct 41 b, the electrons pass through an exit aperture 39 inthe lower plate 42 and are then accelerated towards a display screen(not shown).

The apertures 35 a, 35 b, 35 c in the middle plate 36 are surrounded byelectrodes (not shown in this Figure).

The configuration shown in FIG. 3 is especially suitable for guiding anelectron beam towards one of a tile of picture elements of the displayscreen. In a preferred embodiment of the invention, the picture elementson the display screen are thus grouped into, for example, 4×4 or 8×8tiles. For each tile, an electron beam is generated passing into acorresponding branched network for guiding said electron beam towardsany picture element of the respective tile.

FIGS. 4A to 4C schematically show an arrangement of three plates, amiddle plate 36 in between plates 41 and 42, illustrating the basicprinciple of the invention. FIG. 4A shows the “ideal” situation: Anelectron current 33 is injected through an aperture in a plate (thiscould be an aperture in any of the plates 36, 41 or 42), said plates 41and 42 have apertures provided with electrodes for providing electricalpotentials. The electron current “hops” along the transport ducts formedin and in between the plates 36, 41 and 42, from an input aperture to anoutput aperture, being selectively guided through selection aperturesbetween the input and output aperture. The input current equals theoutput current, and for non-selected output apertures the output currentis zero. This is the ideal situation.

The inventors have, however, found that this is not always the case. Theinput current does not always equal the output current, and non-selectedoutput apertures may show a residual current. This leads to adegradation of the image on the display screen.

It is an object of the invention to improve the image.

FIG. 4B shows that gaps 43 may be formed in between the middle plate 36and plate 41 and/or 42. The inventors have realized that an arrangementwith three plates, in which electron ducts alternately run at eitherside of the middle plate, has as a consequence that over the middleplate a potential along the plate is formed. When a gap or slit inbetween the plates is present, this may cause electrons to tunnel inbetween plates, as is schematically shown in FIG. 4B. “Tunneling”,within the concept of the invention, indicates unwanted transport ofcharge through cracks or slits in between plates.

Electrons can for instance leak to an electrode or to a neighboringtransport duct. Even by means of field emission (e.g. from an electrode)electrons may be emitted and find their way to another electrode ortransport duct. This tunneling has a number of effects, first of allcurrent is lost, since electrons tunnel in between the plates and arelost. Further, some current may find its way from a part of a transportduct at a certain level to a higher level part of a transport duct, evenif this was not the intention. Also, some current may hit the selectionelectrodes causing a current to run in the selection electrodes.

Finally, apart from a diminution of the “intended” current and therebyof the current hitting the display screen at the intended position,there are also currents hitting the screen at unintended positions,after having tunneled in between the plates to a duct which should havebeen closed. The diminution and related effects are caused by randomprocesses and thus are relatively unpredictable, which makescountermeasures difficult.

Measurements on, in this case, three plate arrangements showed that aleakage current can indeed occur from a first selection chamber ortransport duct to the metal tracks of a second, higher order selectionchamber or transport duct. This can occur because the second, higherorder, selection electrodes need to be at a higher voltage in order topull the electrons through the selection apertures. However, if a smallopening is present between the glass plates then the electrons can takea “short-cut” as indicated in FIG. 4B. This means that a part of thecurrent is “lost” in transit, which is an unwanted effect.

The basic concept of the invention is shown schematically in FIG. 4C:the provision of an anti-leakage layer 44 on either the middle 36 or theadjacent plates 41 and/or 42 for providing in operation atunneling-counteracting potential on a plate of the arrangement toprevent tunneling of electrons through gaps between the middle plate andan adjacent plate from one transport duct to another.

In one embodiment the anti-leakage layer(s) is (are) passive layers. Bypassive layers are meant layers that, in operation, obtain atunneling-counteracting potential in a passive manner. At facing sidesthe plates 36, 41 and 42 may be covered with a non-conductive, lowsecondary emission (low δ, i.e. lower than 1) coating, at such positionwhere no hopping transport should take place. A low secondary emissionlayer will, when hit by electrons, quickly acquires a negative electriccharge, thus building a tunneling counteracting potential. This willprevent hopping transport of electrons.

An alternative solution is to cover at least some of the selectionelectrodes with an insulating material, e.g. frit, SiO2, SiN, etc. Thiswill prevent the electrons from reaching the metal tracks, causing theinsulator to charge and a repulsive, thus tunneling counteracting,potential to build up, hence preventing a leakage current. One or bothsuch layers will prevent tunneling in a passive manner, i.e. thepotential will build up “automatically”, with no extra measuresrequired.

The difficulty with the “passive layer” solutions it that the electrodesin and around the selection apertures should not be covered with aninsulator, or else they will charge up as well, inhibiting the veryfunction of the selection electrodes. Thus this will need anotherlithography, shadow evaporation or printing step.

In another embodiment of the invention the anti-leakage layer is formedby an active anti-leakage layer, meaning a conductive layer withconnections for being connected to a potential source, by which means apotential barrier against tunneling of electrons may be made.

In such embodiment the leakage current is prevented by conductivelayers, which could be called “guard electrodes”. These guard electrodesare, in operation, connected to one or more potential sources providingto the guard electrodes a potential lower than the potential of the lastselection electrode. Any electrons straying away from their course arethen driven back, by the guard potential applied to the guard electrode,back to the intended course. This will require at least one extra metaltrack, and means for providing an electric potential.

FIGS. 5A to 5E and 6A to 6E show two embodiments of the invention inwhich guard electrodes (eg) are used. The guard electrodes form astructured anti-leakage layer. The Figures are arranged as follows:

FIGS. 5A and 6A show a planar view of all transport ducts, selectionchamber, selection electrodes and guard electrodes. In these Figuressequential selection apertures are denoted by a, b, c, d and e. a is thefirst (entrance) aperture, e is the final (exit) aperture. One possibleselection path through a sequence of apertures a to e is schematicallyshown by means of an arrow. The guard electrodes are indicated byarrows. Although FIGS. 5A and 6A show all details in their mutualpositions, these Figures are also quite complicated, so FIGS. 5C to 5Eand 6B to 6E are drawn to show details.

FIGS. 5C and 6B show the arrangement of the transport duct, selectionchambers and selection apertures a to e; in these Figures the possibleselection path is also shown. An electron stream enters through aperturea, is then directed through selection apertures b by applying suitablepotentials to selection electrodes eb (see FIGS. 5C, 6C) either to theleft or to the right (in the shown path to the left), is transportedthrough a transport duct to a next selection stage c (comprising twoselection apertures c), is directed (by applying suitable potentials toselection electrodes ec (see FIGS. 5D, 6D) surrounding the selectionapertures c) up or down (in the downward path shown), is transportedthrough a transport duct to a next selection stage d, is then directedthrough selection apertures d by applying suitable potentials toselection electrodes ed (see FIGS. 5C, 6C) either to the left or to theright (in the shown path to the left), and is finally directed, byapplying suitable potentials, to electrodes ee (see FIGS. 5E and 6E) toan exit aperture e.

FIGS. 5C and 6C show the arrangement of the selection electrodes eb (forselection apertures b) and ed (for selection electrodes d). Theseselection electrodes are provided on one side of one plate. FIG. 5Cshows that, apart from the selection electrodes eb and ed, a further setof electrode eg is provided on the same surface. These form the guardelectrodes, when applied with a potential lower than the potential ofelectrodes d, these guard electrodes form a potential barrier forelectrons in a selection chamber comprising the selection apertures c,from “hopping along” the surface to the electrodes d. Unwanted leakageof electrons through gaps in between the plates from one selection stage(stage c) to the next (stage d) is thereby prevented.

In FIG. 6C the b-electrodes eb are arranged in such manner that theythemselves function as guard electrodes. In this embodiment thereforethe electrodes from a first selection stage (b) act as the guardelectrodes for a second selection stage (c). This decreases the numberof voltages that need to be applied, but switching the first selectionhas a larger effect on the hopping of the electrons from the firstselection to the second selection. This is an example of a preferredembodiment of the invention in which the electrodes are arranged suchthat they have a double function, forming selection electrodes near oneor more apertures, and anti-leakage layers (guard electrodes) at anotheror other positions. This reduces the number of connections, voltages andelectrodes needed.

FIGS. 5D and 6D show the selection electrode ec for the c-selectionapertures. They are arranged on a surface of a plate different from thesurface in which the b and d electrodes (FIG. 5C, 6C) are arranged. Inthese Figures guard electrodes eg are also shown, they prevent unwantedleakage of electrons from stage b to stage c.

Finally FIGS. 5E and 6E show the exit selection electrodes ee, which arearranged in one plane near the exit side of the exit electrodes which isa plane different from the plane for the electrode eb and ed, and theplane for the electrodes ec.

Of course it may be advantageous to combine the solutions of thedifferent embodiments. This is in particular the case if the selectionelectrodes act as field emitters, because the guard electrode will notor only partially prevent electrons from hopping between one selectionelectrode and the next once they have been emitted by such a fieldemitter.

In the embodiments described, the network is two-dimensional, the exitsform a two-dimensional array and the nodes of the network also form atwo-dimensional array. The term two-dimensional is to be understood tomean herein that, viewed in projection on the display screen, thecurrent distribution is two-dimensional. A network according to theabove shown embodiments of the invention could be described as being atwo-dimensional current distributor, having electron ducts,interconnected at nodes, which nodes form junctions of at least oneentrance and at least two exhaust ducts, whereby at each node, anelectron current coming in through the entrance duct can, by means ofapertures connecting the entrance and exhaust ducts, be steered into adesired exhaust duct.

In the examples shown above, the electron currents move, apart from thefeedthroughs between the transport ducts, in the transport ducts in thehorizontal and/or vertical direction. The invention is not limitedthereto. The network can be three-dimensional and comprise transportducts in three or more directions. In the examples, the image displayedis two-dimensional. The invention is not limited thereto, the imagedisplayed can be three-dimensional, for example, it can be displayed onthe sides of a cube. It is alternatively possible to display the imageon the surface of a hemisphere, the network being constructed so as toform a number of hemispheres which are stacked on top of each other, andeach hemisphere comprising transport ducts.

The wording in the claim that “a” or “an” element is present includesthe possibility that a plurality of such elements is present. Forinstance, the display device may comprise a single branched network thatdistributes electrons from the electron source over the entire displayscreen. However, preferably the number of branched networks is largerthan one, whereby a branched network distributes electrons over anassociated portion of the display screen. Each branched network has itsown entrance and a corresponding electron source.

In summary, a display device (1) comprises a luminescent display screen(15), and means for directing electrons towards the display screen (15).Said means comprise an arrangement of at least three plates (36, 41,42), with a middle plate (36) having selection apertures, and selectionelectrodes associated with the selection apertures. The selectionapertures, the selection electrodes and the plates are arranged forhaving the electron currents, on their way from a source to theelectroluminescent screen, selectively pass the apertures in the middleplate and alternately run at opposite sides of the middle plate. Ananti-leakage layer (44, eg) for providing, in operation, a tunnelingcounteracting potential is provided on a plate of the arrangement toprevent leakage of electrons through gaps between the middle plate andan adjacent plate.

1. A display device (1) comprising: a vacuum envelope, an inner side ofwhich is provided with a luminescent display screen (15), said vacuumenvelope comprising at least an electron source (31) for emittingelectrons and directing means for directing the electrons towards saiddisplay screen (15), said directing means comprising: an arrangement ofat least three plates (36, 41, 42) defining a network (10) of electrontransport ducts, a middle plate (36) of the arrangement being providedwith selection apertures and selection electrodes associated with theselection apertures for application of selection voltages, saiddirecting means being arranged for selectively passing the electronsthrough the selection apertures and directing the electrons alternatelyat opposite sides of the middle plate, characterized in that ananti-leakage layer (44, eg) is provided for enabling, in operation, anelectric potential on a plate of the arrangement, said electricpotential counteracting tunneling of electrons between the middle plateand an adjacent plate.
 2. A display device as claimed in claim 1,characterized in that the anti-leakage layer is formed by a passiveanti-leakage layer.
 3. A display device as claimed in claim 2,characterized in that facing sides of the plates (36, 41 and 42) areprovided with a non-conductive coating having a secondary electronemission coefficient smaller than 1, at positions where no electrontransport should take place.
 4. A display device as claimed in claim 2,characterized in that at least some of the selection electrodes are atleast partly covered with an insulating material.
 5. A display device asclaimed in claim 4, characterized in that all selection electrodes areat least partly covered with the insulating material.
 6. A displaydevice as claimed in claim 1, characterized in that the anti-leakagelayer is formed by an active anti-leakage layer (eg).
 7. A displaydevice as claimed in claim 6, characterized in that at least some of theselection electrodes are arranged for functioning as the anti-leakagelayer.